Non-enhanced magnetic resonance angiography (NE-MRA) is a medical imaging technique used in the evaluation of vascular disease, especially in patients with impaired renal function. Most conventional NE-MRA techniques rely on flow properties of blood to generate sufficient contrast between blood-vessels and background for effective visualization of pathology. For example, during many clinical assessments, it is necessary to accentuate the arteries while suppressing the veins and all other tissues. The reliance on blood-flow presents challenges to contrast manipulation, considering that the flow patterns are different from subject to subject, and may be altered by vascular pathologies such as stenosis. Moreover, irregular heart rhythms can also affect flow, leading to sub-optimal contrast between arteries and other tissues.
An alternative approach that does not rely on blood-flow uses the natural contrast of blood and tissues to depict the pathology. However, the presence of background tissues and veins makes interpretation of images challenging for a clinician. Consequently, such an approach is not used traditionally in NE-MRA.
As a trade-off, in most conventional NE-MRA applications, blood-flow is used to manipulate contrast and extra time is built-in to permit sufficient inflow of blood, as well as to account for differences in blood-flow among subjects and differences between healthy vasculature and pathology. This extra time introduces inefficiency in the overall magnetic resonance (MR) acquisition process as the system remains idle which blood inflow occurs. Moreover, if the flow mechanism is compromised for any reason, the process must be repeated. Accordingly, it is desired to use the blood inflow time to acquire an additional set of images that has no reliance on blood flow (i.e., a flow-independent image set) that offers a back-up in the event that the flow mechanism is compromised for any reason.